This invention relates to magnetic field detectors and, more particularly, to detectors utilizing compositions for visible indication of the presence of a magnetic field.
A well-known class of magnetic field detectors is the compass family which utilizes a needle which aligns with the earth's magnetic field to provide an indication of direction relative to indicia on the compass.
In many situations, it is desirable to learn merely by visible indication whether a magnetic field is present or whether the magnetic field is one of a particular spatial configuration. For example, a visible indication of whether a magnetic tape is in the recorded or erased state is oftentimes desirable. Furthermore, in processes where the absence of a magnetic field is required such as in the cooling step of thermoremanent erasure of a magnetically recorded tape it is desirable to determine simply and visibly that the cooling tape is not being subjected to an applied magnetic field.
U.S. Pat. No. 3,013,206 to Youngquist et al. discloses a magnetic reader comprising a hollow non-ferromagnetic vessel having a cavity filled with a suspension of flat, visible, weakly ferromagnetic crystals which orient when suspended in the liquid and in response to a magnetic field.